The present invention relates to a power conditioning unit, in particular a power conditioning unit for low-power grid-connected applications.
Power converters for photovoltaic modules in the range of 100-300 W have been reported over the past 10 years. For example, “Design optimisation of a single phase inverter for photovoltaic applications” (S B. Kjaer, F. Blaabjerg, IEEE Power Electronics Specialist Conference, Vol. 3, pp. 1183-1190, June 2003), “A flyback-type single phase utility interactive inverter with low-frequency ripple current reduction on the DC input for an AC photovoltaic module system” (T. Shimizu, K. Wada, N. Nakamura, IEEE Power Electronics Specialist Conference, Vol. 3, pp. 1483-1488, June 2002), “Inverters for single-phase grid connected photovoltaic systems—An overview” (M. Calais, J. Myrzik, T. Spooner, V. G. Agelidis, IEEE Power Electronics Specialist Conference, Vol. 4, pp. 23-27, June 2002) and “A novel ac photovoltaic module system based on the impedance-admittance conversion theory” (S. Yatsuki, K. Wada, T. Shimizu, H. Takagi, M. Ito, IEEE Power Electronics Specialist Conference, Vol. 4, pp. 17-21, June 2001).
A wide range of power electronics topologies are suggested in the literature and some are readily available in the market or patented. EP0780750, JP10174452, U.S. Pat. No. 5,898,585, DE19732218, EP0947905, JP2000020150, JP2000324852 and JP2003289674 are examples. However, it has been observed that the performance offered by such devices is far from optimal. In some instances the lifetime of the PCU is too short compared to the lifetime of the PV panel, in other cases the cost is excessive, and in others the efficiency is low. It is also reported that there is a compromise among these three factors. The PCU we describe offers a low-cost solution while maintaining a relatively high efficiency and reliability. The system overall cost is reduced by introducing a simple control of the current injection into the grid that uses generic logic gates and operational amplifiers.